To get the best from those homegrown herbs, you have to
harvest them when the oils
responsible for their flavor and aroma are at their peak. Timing
these flavor peaks
depends on the plant part you’re harvesting and how you intended
to use them.

Herbs grown for their foliage should be harvested before they
flower. While chives are
quite attractive in bloom, flowering can cause the foliage to
develop an off-flavor.
Harvest herbs grown for seeds as the seed pods change from green
to brown to gray but
before they open. Collect herb flowers, such as borage and
chamomile, just before full
flower.

Some guidelines:

• Begin harvesting a foliage herb when the plant has enough
  foliage to maintain growth.
  You can pick up to 75 percent of the current season’s growth
  at one time.
• Harvest early in the morning, after the dew dries, but
  before the heat of the day.
• Pick leaves before flowering. Otherwise, leaf production
  declines.
• Herb flowers have their most intense oil concentration and
  flavor when harvested after
  flower buds appear but before they open.
• When picking herb flowers to dry for craft purposes, pick
  them just before they’re fully
  open.
• Annual herbs can be harvested until frost.
• Perennial herbs can be clipped until late August in north
  Georgia and late September in
  south Georgia. Stop harvesting about one month before the
  frost date. Late pruning could
  encourage tender growth that can’t harden off before
  winter.